1. Field of the Invention
The present invention relates to a quartz crystal oscillator for supplying an oscillating frequency signal and which includes a quartz crystal unit and an IC (Integrated Circuit) chip that together with the crystal unit forms an oscillation circuit; and more particularly to a surface-mount crystal oscillator which is suitable for miniaturization wherein a mounting substrate, on which an IC chip is provided, is bonded to the rear surface of a crystal unit.
2. Description of the Related Art
Surface-mount crystal oscillators, and in particular, surface-mount temperature-compensated crystal oscillators (TCXOs) feature light weight, compact size, and an oscillation frequency having superior stability, and these devices are therefore widely used in communication equipment such as portable telephones which are used in a mobile environment. As one type of surface-mount crystal oscillator, there is a bonded-type surface-mount crystal oscillator in which a mounting substrate, on which an IC chip constituting an oscillation circuit has been mounted, is installed on the rear surface of a crystal unit.
FIGS. 1A and 1B show an example of the construction of this type of the conventional surface-mount crystal oscillator, and FIGS. 2A and 2B show a crystal unit that is used in this surface-mount crystal oscillator. The surface-mount crystal oscillator is provided with crystal unit 1 and mounting substrate 2. Crystal unit 1 is constructed by accommodating quartz crystal blank 3 within casing 4 and covering the opening of casing 4 by metal cover 5.
Casing 4 is shaped with a depression or recess formed in one of its principle surfaces, and is constructed from laminated ceramic in which bottom wall 4a and frame walls 4b are stacked together. Bottom wall 4a is a substantially rectangular planar shape, and frame walls 4b are formed as a substantially rectangular frame shape, whereby a substantially rectangular depression is formed in casing 4. As shown in FIG. 2A, a pair of terminal electrodes 6 is formed on the bottom surface of the depression of casing 4, i.e., the exposed surface of bottom wall 4a. 
Crystal blank 3 is, for example, a substantially rectangular AT-cut quartz crystal blank, and although not shown here, excitation electrodes are formed opposite each other on the two principal surfaces of the crystal blank. In addition, extension electrodes are formed on both sides of one end of crystal blank 3 to extend from the excitation electrodes. Both sides of one end of crystal blank 3 from which the extension electrodes extend are secured to terminal electrodes 6 on the bottom surface of the depression of casing 4 by means of conductive adhesive 12. Crystal blank 3 is thus held horizontally and connected electrically and mechanically to casing 4.
A pair of connection terminals 7a are provided at diagonally opposite corners on the outer side of the bottom surface of casing 4. These connection terminals 7a are connected to respective terminal electrodes 6 by way of conductive paths formed in casing 4. In addition, ground terminals 7b are provided at the other diagonally opposite corners on the outer side of the bottom surface of casing 4. Ground terminals 7b are electrically connected to metal cover 5. Typically, metal cover 5 is bonded to casing 4 by providing a metal ring on the upper surface of frame walls 4b and then seam welding metal cover 5 to this metal ring.
As with casing 4, mounting substrate 2 is composed of laminated ceramic and has a depression or recess formed in one of its principal surfaces. The planar outer dimensions of mounting substrate 2 are greater than the planar outer dimensions of crystal unit 1, and in particular, are set to allow a space at one longitudinal end of mounting substrate 2 that is not covered by crystal unit 1 when crystal unit 1 is bonded to mounting substrate 2. In addition, connection terminals 8a which correspond to connection terminals 7a of crystal unit 1 and ground terminals 8b which correspond to ground terminals 7b of crystal unit 1 are provided on the surface of mounting substrate 2 which is bonded to crystal unit 1, i.e., on the surface in which the depression is not formed. Then, for example, two chip capacitors 9 are mounted in the space at the longitudinal end of mounting substrate 2.
Mounting terminals 10 including terminals such as a power supply terminal, a ground terminal, an output terminal, and an AFC (Auto Frequency Control) terminal are provided on the upper surface of frame part 2a which surrounds the depression of mounting substrate 2. IC chip 11 is secured inside the depression, by the way of, for example, ultrasonic thermocompression using bumps. An oscillation circuit which connects to crystal unit 1 and a temperature compensation mechanism for effecting temperature compensation of the oscillator frequency of the oscillation circuit are integrated on IC chip 11. Mounting terminals 10 are electrically connected to the respective terminals of IC chip 11. IC chip 11 is also electrically connected to connection terminals 8a and ground terminals 8b. 
Mounting substrate 2 is installed on the rear surface of crystal unit 1 by bonding connection terminals 7a and ground terminals 7b of crystal unit 1 to connection terminals 8a and ground terminals 8b of mounting substrate 2 by means of, for example, solder.
As chip capacitors 9, a variety of large-capacitance capacitors which are difficult to incorporate inside IC chip 11 may be arranged, including bypass capacitors between power supply and ground, capacitors for coupling with a succeeding stage, or CR filter capacitors for suppressing noise which is produced by the temperature compensation mechanism.
In a surface-mount crystal oscillator of the above-described construction, however, there is the problem that, with the development of ever-smaller devices, the space for mounting chip capacitors 9 becomes difficult to maintain, such as in a case in which the planar outer dimensions of the oscillator are reduced to a size of 3.2 mm×2.5 mm.
This problem does not arise if crystal unit 1 becomes correspondingly smaller with the decrease in the planar outer dimensions. Decreasing the size of crystal unit 1 entails a choice between either correspondingly reducing the outer dimensions of crystal blank 3 or leaving the size of crystal blank 3 unchanged and increasing the ratio of the inside volume of casing 4 to the outer dimensions of the crystal oscillator. However, since the oscillation characteristics of a crystal blank improve with the increase in the outer dimensions of the crystal blank, decreasing the outer dimension of crystal blank 3 has an adverse effect on the oscillation characteristics of crystal blank and thus results in design problems. More specifically, decreasing the dimensions of crystal blank 3 brings about an increase in the crystal impedance (CI) of crystal blank 3 and the occurrence of spurious oscillation. On the other hand, increasing the ratio of the inside volume of casing 4 necessitates a corresponding decrease of the thickness of frame walls 4b. Frame walls 4b are composed of ceramic, and reducing the thickness not only reduces strength, but also raises problems in fabrication. Frame walls 4b must maintain a particular fixed thickness. Thus, in either case, reducing the size of crystal unit 1 is problematic, and a reduction of the outer dimensions of mounting substrate 2 thus prevents mounting of chip capacitors 9.